1. Field of the Invention
The invention relates to the field of applied thermodynamics, and in particular to reversing valves for reversible heat pump systems.
2. Description of the Related Technology
Heat pump systems use a refrigerant to carry thermal energy between a relatively hotter side of a circulation loop to a relatively cooler side of the circulation loop. The refrigerant is most compressed at the hotter side of the loop, where a compressor raises the temperature of the refrigerant. Evaporation of the refrigerant occurs at the cooler side of the loop, where the refrigerant is allowed to expand, thus resulting in a temperature drop. Thermal energy is added to the refrigerant on the cooler side of the loop and extracted from the refrigerant on the hotter side, due to the temperature differences between the refrigerant and the indoor and outdoor mediums, respectively, to make use of the outdoor mediums as either a thermal energy source or a thermal energy sink. A reversible heat pump system is essentially an air conditioner that contains a reversing valve that lets it switch between “air conditioner” and “heater.” When the reversing valve is switched one way, the heat pump acts like an air conditioner and the inside coil or heat exchanger is cooled, and when the reversing valve is switched the other way it reverses the flow of refrigerant and the inside heat exchanger is heated.
Residential air to air reversible heat pump systems are bidirectional, in that suitable valve and control arrangements selectively direct the refrigerant through indoor and outdoor heat exchanger coils so that the indoor heat exchanger is on the hot side of the refrigerant circulation loop for heating and on the cool side for cooling. A circulation fan passes indoor air over the indoor heat exchanger and through ducts leading to the indoor space. Return ducts extract air from the indoor space and bring the air back to the indoor heat exchanger. A fan likewise passes ambient air over the outdoor heat exchanger, and releases heat into the open air, or extracts available heat therefrom.
These types of heat pump systems operate only if there is an adequate temperature difference between the refrigerant and the air at the respective heat exchanger to maintain a transfer of thermal energy. For heating, the heat pump system is efficient provided the temperature difference between the air and the refrigerant is such that the available thermal energy is greater than the electrical energy needed to operate the compressor and the respective fans. For cooling, the temperature difference between the air and the refrigerant generally is sufficient, even on the warmest days.
Heat pumps systems can be extremely efficient in their use of energy. However, one problem with most heat pumps is that the heat exchanger coils in the outside air may collect frost and ice. The speed of the frost build-up is strongly dependent on the ambient temperature and the humidity ratio. Coil frosting results in lower coil efficiency while affecting the overall performance (heating capacity and coefficient of performance) of the unit. The heat pump has to melt this ice periodically, so most conventional reversible heat pump systems will temporarily switch back to air conditioner mode, even in the dead of winter, to heat up the coils. This is also known as refrigerant cycle inversion. To avoid pumping cold air into the house in air conditioner mode, the heat pump also typically activates a backup electrical or fossil fuel burning heat source to heat the cold air that indoor heat exchanger creates when the refrigerant cycle is inverted. Once the ice is melted, the heat pump switches back to heating mode and turns off the backup source of heat. In most residential heat pump systems, the source of backup heat is electrical resistance heating, which is expensive and very energy intensive.
Coil defrosting using the refrigerant cycle inversion technique negatively impacts the overall efficiency of the reversible heat pump system unit because the hot refrigerant in the unit that provides the desired heat is actually cooled when the refrigerant cycle is inverted. Moreover, interrupting the operation of the compressor is unhealthy to the compressor and requires waiting several minutes before its operation can be resumed. Frequent interruption of the system also tends to reduce the useful life of the compressor and the fan.
U.S. Pat. No. 6,491,063 to Benatav discloses an air conditioning system having a rotary change-over valve that can be operated to shunt a part of the refrigerant from the high pressure port of the compressor to the low pressure port to thereby control temperature within the system without interrupting the compressor. Another described additional function is to restrict the effective cross-sectional area of the low pressure port with respect to the heat-exchanger connected to it, to thereby control the output of the system without interrupting the operation of the compressor. A further control function is to selectively open and close the pilot valve, not only for making a change-over operation, but also for controlling leakage from the high pressure port to the low pressure port for temperature control purpose in any position of the valve. The reference states that the disclosed system can be operated to prevent frosting. It states that the shunting of refrigerant could be performed periodically by periodically controlling the amplitude of the leakage, the time interval of each period of leakage, and/or the frequency at which the leakage is effected. It also discloses that the leakage may be continuous, wherein a continuous leakage could be provided having a magnitude depending on the output of the temperature sensor to prevent frosting, that it could be controlled manually or automatically in response to temperature.
The temperature of the heat exchanger at the time at which defrosting is initiated, the time interval for which the defrosting is conducted and the final temperature of the heat exchanger at the end of the defrosting interval impacts the overall efficiency of the heat pump system whether the refrigerant cycle inversion method or the shunt method disclosed in Benatav is used. Of particular importance to system efficiency is the heat exchanger temperature at which defrosting is initiated, and the relationship of that temperature to the surrounding air temperature and possibly the humidity of the air. The Benatav system fails to take such criteria into account when determining defrost cycle control.
A compressor's discharge temperature is often overlooked when troubleshooting faulty heat pump systems. It is typically not taken into account when factoring system efficiency, or correcting or altering system performance during the run cycle. However, compressor discharge temperature is very important because it indicates the amount of heat absorbed in the evaporator and suction line, plus any heat generated by the process of compression. Because the compressor's discharge temperature is superheated, a pressure-temperature relationship does not exist. The discharge temperature must be read directly on the discharge line at 1″ to 2″ from the compressor. The discharge temperature should never exceed 225 degrees F., since higher temperatures will carbonize and breakdown refrigeration oils, which are needed to lubricate the compressor. Sustained high temperatures can also damage other components of the compressor. The three causes of high discharge temperature are: High Condensing Temperature; Low Evaporator Temperatures and Pressures; and High Compression Ratios. As anyone familiar with the art understands these terms, their common causes, properties and relationships, they will not be discussed here for the sake of mere definition, except that: Compression Ratio=Absolute discharge pressure divided by Absolute suction pressure. (e.g. 400 psi discharge/100 psi suction=4:1 Compression Ratio.)
A need exists for an improved reversible heat pump system and a method of operating such a system that is more efficient than conventional systems and that is economical to produce, install, and retrofit into existing systems and to operate.